Osteonecrosis of the humeral head in an adolescent without clear pathogenesis has not been reported in the literature. In this case report, we present such a case of humeral head osteonecrosis in a 15-year-old adolescent. The lesion was located at the subchondral area of the medial part of the humeral head with characteristic appearances on MRI. The shoulder was immobilized in a sling until the pain disappeared, and the patient was told to refrain any kind of sport activities. Bone remodeling was noted five months after the first visit, and it took 2 years for the lesion to be totally healed.